Kidney fibrosis is the hallmark of chronic kidney disease (CKD). Despite aggressive management, CKD often progresses to end-stage renal disease, which costs the US >$40 billion dollars and >90,000 deaths annually. The current main therapy targeting the renin-angiotensin-aldosterone (aldo) system with drugs including Spironolactone often delays, but does not stop the progression. This is also true for all other drugs such as endothelin 1 (ET1) blocker Atrasentan. The ineffectiveness and side effects including hyperkalemia and edema necessitate identification of novel therapeutic targets for the development of more effective treatments. Factors modulating the aldo global effect from its primary action site connecting tubule/collecting duct (CNT/CD) may prove better targets. However, such genetic and epigenetic factors remain virtually unknown, partially because of the intrinsic limitations of the clinical studies. These limitations include lack of kidney biopsies to verify the status of the disease, impossibility of genetic manipulation in patients to establish th causative relationship, and impracticability through mutational analyses with blood DNA to identify somatic mutations, which occur at atypical high rate in human kidney. Our published and preliminary data suggest that 1) Patients with diabetic nephropathy (DN) and CKD may have mutations in histone H3 K79 methyltransferase hDOT1L and abolished H3 dimethylation (H3m2K79) in their kidney biopsies; 2) Dot1a (encoded by Dot1l) represses ET1 and other aldo target genes. Aldo relieves Dot1a-mediated repression by multiple mechanisms; 3) CNT/CD-specific ablation of Dot1l in Dot1lAC mice causes abolition of H3m2K79, upregulation of ET1, and development of severe kidney fibrosis throughout the whole kidney. Accordingly, in this proposal, we will develop genetic markers to overcome the above limitations. To this end, we will use kidney biopsies from patients with DN and CKD, our mouse models bearing intact or disrupted Dot1l and ET1 in the CNT/CD, and their CNT/CD primary cells in combination of cutting-edge technologies including laser capture microdissection, next generation sequencing, and in vivo lineage tracing. Our specific aims are to study if DN and CKD patients have genetic defects in hDOT1L (Aim 1), study if Dot1l deletion accelerates kidney fibrosis in part by upregulating ET1 in mice (Aim 2), and study if Dot1a and ET1 modulate the global effect of aldo profibrotic action (Aim 3). Our studies may identify DOT1L as a novel repressor of ET1 and thus a new renoprotective factor, confirm loss of DOT1L function and thus H3m2K79 as an epigenetic driver of CKD, define Spironolactone + Atrasentan as a new effective combinational therapy of CKD, and lay the foundation of new genetic tests. If Dot1l and ET1 are genetically linked to CKD in humans, they may be exploited to develop genetic screening tests to identify patients at high risk of CKD and to determine their responsiveness to various aldo and ET1 inhibitors. Like ET1, DOT1L can also be considered as a potential new therapeutic target of CKD.